The present invention relates to a method and an apparatus for electrochemical detection and more particularly to a method and an apparatus for electrochemical detection of solutions containing at least one substance to be analyzed by means of an electrochemical detector. An electrochemical detector may have a detector cell comprising an inlet and an outlet for an electrolytic solution to be analyzed. A working electrode is in contact with the solution which flows through a solution flow space. A counter-electrode arrangement is provided by means of which an electric potential with respect to the working electrode can be impressed on the solution. The foregoing potential is capable of effecting the electrolysis of the substances in the solution which are to be analyzed at the working electrode.
An important field of use of electrochemical detection and electrochemical detectors, respectively, is the field of liquid chromatography. In this method of analysis the substance to be analyzed is added to a carrier liquid which is supplied by a pump, and the resulting solution is passed onto an analytical column. This column has a retention effect. This means that different substances of the test mixture in the solution are retained on the column for different amounts of time. The individual substances of the test mixture then arrive successively with respect to time at the exit of the column and may thus be analyzed individually.
The analysis may be performed by means of an electrochemical detector comprising a detector cell having the initially indicated features. Preferably, the solution flow space of this detector cell is designed such that the electrolytic solution flowing therethrough assumes the shape of a thin-layer. An example of a chemical detector cell of this type is shown in FIG. 1 which is taken from the publication "Detectors for Trace Organic Analysis by Liquid Chromatography: Principles and Applications," from Vo. 2 ADV. ANAL. and CLINIC. CHEM., PLENUM, NEW YORK, '78 and is hereby incorporated by reference. The electrolytic solution coming from the column is passed through a plastic tube to an inlet, flows through a thin-layer solution flow space, into which a working electrode extends, and leaves the detector cell through an outlet. From there the electrolytic solution flows through an additional plastic tube into a housing containing a potential measuring electrode (usually known as "reference electrode") by means of which the electric potential of the electrolytic solution is measured.
The electrolytic solution flows from this housing through a discharge line to a discharge means for the electrolytic solution. A part of the discharge line is formed by a short metal tube piece which is used as an auxiliary electrode via which a potential is applied to the electrolytic solution. The auxiliary electrode is necessary since the usual measuring electrodes are not current-resistant. The current which flows off via the working electrode due to an oxidation or reduction of the electrolytic solution is therefore supplied via the auxiliary electrode. As stated in the above-mentioned publication, the potential difference between the electrolyte and the working electrode, which is necessary for ionizing the electrolytic solution, is brought about by maintaining the working electrode at ground potential and by bringing the electrolyte to the required potential difference with respect to the working electrode by means of the auxiliary electrode. For this purpose, the electrolyte potential is measured by means of the measuring electrode and the potential of the auxiliary electrode is controlled with the aid of this measured value such that there is the desired potential difference between the electrolyte and the working electrode.
The current generated at the working electrode during the oxidation or reduction may be converted to a proportional voltage which, in turn, may be applied to the input of a measurement value recorder.
This recorder draws a chromatogram which shows some basic signal value (usually also called a "base line") and peaks (sometimes referred to as measurement signal peaks) standing out therefrom, the position of which on the time axis of the chromatogram depends on the type of the substances in the electrolytic solution that are to be analyzed and the height of which depends on the concentration of the substances to be analyzed. A noise and interference voltage is superimposed on the basic signal value, and the amplitude of that voltage creates a resolution limitation for the chromatogram. Thus, great efforts are made in order to keep the noise component of the basic value signal as low as possible, so as to be able to analyze substances in very small amounts.
It is known from the article "Optimierung des Signal/Rausch-Verhaltnisses bei der elektrochemischen Detektion der Katecholamine in Plasma- und Urinproben" (Optimization of the Signal-to-Noise Ratio in the Electrochemical Detection of Catecholamines in Plasma and Urine Samples) by W. Bauersfeld and H. Wissner, published in the collection "Konigsteiner Chromatographietage, Oct. 4 to 6, 1982, Travemunde/Ostsee" (seminar of the firm Waters), to accommodate the detector cell in a Faraday cage on the one hand, and on the other hand to use an additional filter for the detector electronics for the purpose of reducing the noise component of the chromatogram. In this manner, it was possible to achieve a reduction of the noise down to a peak-to-peak value of 2 pA.sub.ss, which according to that article was not attained before. This value is shown in the chromatogram in FIG. 2 which is taken from the afore-mentioned publication.
It is thus desirable to provide a method and an apparatus for electrochemical detection by means of which the noise component in the chromatogram can be reduced considerably and the analysis sensitivity can thus be increased.